This invention pertains generally to an improved design for nozzles used in the generation of plasmas and more particularly to an improved design for reducing nozzle erosion in proximity to energetic plasmas.
The generation and use of extreme ultraviolet (EUV) or soft x-ray radiation i.e., light whose wavelength in the range 3.5-15 nm, has wide applicability in the fields of materials science, microlithography and microscopy. Two frequently used sources of such radiation are a laser-produced plasma and synchrotron radiation. With appropriate modification laser plasma sources are as bright as their more expensive synchrotron counterparts and are better suited to a small laboratory or commercial environment. However, typical laser plasma sources using solid metal targets suffer from the disadvantage that they generate particulate ejecta that can damage and/coat nearby optical surfaces to their detriment.
As described in U.S. Pat. No. 5,577,092, incorporated herein in its entirety, a scheme has been developed for generating ultra-low debris laser plasma targets by free-jet expansion of gases. It is well known to those skilled in the art, that the supersonic expansion of a gas, under isentropic conditions, through a nozzle from a region of high pressure to one of lower pressure causes the temperature of the gas to drop. As the temperature of the gas drops the relative intermolecular velocity of the gas decreases and the weakly attractive van der Waals forces that exist between molecules cause condensation of the expanding gas with the subsequent formation of molecular clusters, for example dimers, polymers and eventually droplets. The formation of molecular clusters is a crucial element in efficient laser absorption, subsequent laser heating and EUV radiation production. These clusters, aggregates of atoms or molecules, will respond locally like microscopic solid particles from the standpoint of laser plasma generation. Each cluster has an electron density well above the critical density necessary for efficient absorption of laser energy. In the absence of these clusters, the density of the gas jet at distances 10-30 mm from the orifice is so low that laser energy is not absorbed and a plasma will not be formed.
As shown in FIG. 1 in the above-referenced U.S. patent, hot, dense plasmas that are a source of EUV radiation are produced by high power laser interaction with small gas clouds, or clusters, formed by the aforementioned supersonic expansion of gas through a nozzle (free-jet expansion) into a vacuum chamber. In addition to the fact that in operation it yields many orders of magnitude less debris than more conventional laser plasma sources, this particular method of forming laser plasma sources has a long life of uninterrupted operation by virtue of the fact that periodic replacement of spent target materials, such as metal tape or drum targets, or cleaning and/or replacement of optical components is not required, inexpensive target materials may be used, there is an almost continuous supply of target materials and it permits laser focus far from the nozzle orifice further reducing debris.
While the use of molecular gas clusters has proven beneficial in reducing deposition of debris onto nearby optical surfaces and thus prolonging their useful life it has been found that energetic particles produced by the plasma cause erosion of nearby plasma-facing bodies, such as the surface of the exit end of the nozzle used to produce the gas clusters. The erosion of the plasma-facing parts of the nozzle is undesirable for two reasons: 1) the eroded material deposits on nearby optical surfaces decreasing their reflectance efficiency in the desirable EUV region of the spectrum thereby decreasing their useful life and 2) erosion changes the nozzle shape thereby affecting the ability of the nozzle to form molecular gas cluster laser targets having the desired properties. What is needed is a method for reducing erosion of the plasma-facing part of nozzles used to form the molecular gas clusters that are the source of the EUV radiation emitting plasma.